1. Field of the Invention
The present invention relates to a tundish impact pad and more particularly to a tundish impact pad that stabilizes the flow of liquid metal exiting the pad.
2. Description of the Related Art
Liquid metal, and in particular liquid steel, is often poured from one vessel to another vessel. For example, liquid metal may be poured from a furnace into a ladle, and or from a ladle into a tundish, and or from a tundish into a mold. When liquid metal is poured into the tundish from the ladle, it is normally poured into the tundish through an outlet in the ladle bottom. The stream from the ladle is metered by a valve and the outlet stream may be enclosed in a ceramic tube, called a ladle shroud, which is connected to the valve.
A typical tundish has a simple design consisting of either a trough or box shaped vessel having a generally horizontal or flat bottom with vertically arranged walls. In these designs, the stream pouring from the ladle, i.e. incoming ladle stream or flow, enters the tundish, impacts the tundish bottom, and spreads in all directions. A flat impact pad is commonly used on the tundish bottom in the impact area pad to reduce erosion of the tundish refractory lining.
After the incoming flow is spread, a portion of the incoming flow rises up the vertical walls of the tundish, travels back along the surface of the liquid steel toward the ladle stream inlet location, and is re-entrained within the incoming flow of the ladle stream or flow. Another portion of the flow does not directly impact the tundish vertical walls and is dispersed throughout the tundish volume.
The aforementioned flow patterns result in many problems. The problems encountered with the above described flow pattern include:
1. Non-separation of slag and inclusion particles. The turbulence introduced by the incoming ladle stream or flow and the pattern of liquid metal flow generated within the tundish does not allow the separation by flotation of buoyant slag and inclusion particles entrained within the liquid metal and can actually cause slag to be re-entrained.
2. Smooth flow disturbance. Turbulence within the tundish caused by the dissipation of the kinetic energy of the ladle stream is propagated above adjacent tundish nozzles and this energy disturbs the smooth flow which is required to properly fill the molds.
3. Thermal inhomogeneity. Short-circuit flow and different liquid metal residence time behavior associated with each tundish to mold stream results in thermal inhomogeneity of the liquid metal contained in the tundish. Tundish exit streams therefore experience different temperatures, with colder metal exiting the tundish furthest from the ladle stream and hotter metal exiting the tundish closest to the ladle stream.
Tundish impact pads having complex geometries have been proposed to alleviate the above problems, but without success. Examples of these pads are disclosed in U.S. Pat. No. 5,169,591 (the "'591 patent") and U.S. Pat. No. 5,358,551 (the "'551 patent"). Both of these patents describe impact pads which contain the inlet flow from the ladle. This is not unique since flow from a ladle has been contained within many different impact pad designs for many years. Moreover, the existence of a continuous wall around a pad with upward release of the flow has been practiced in many designs prior to the existence of the aforementioned patents.
The '551 and '591 patents teach a reversal of the flow generated by the incoming ladle stream. One of the many problems with these pad designs is that they do not address the very real issue of what happens to the flow when the incoming stream is not directed at the exact geometric center of the pad. This is the normal state of affairs in a tundish as the ladle stream moves in practice as the ladle valve compensates for the changing head pressure in the ladle. A non-central location of the incoming ladle stream causes an amplification of the reversed flow and can result in excessive splash or ultimately, liquid metal being ejected from the tundish.
Another problem with these pad designs is that the flow is directed from the pad in an inward and upward manner. This inward and upward flow accelerates the flow and causes it to "rebound" off the surface of the liquid metal in the tundish, causing short-circuit flow to the closer tundish exit strands.
Thus, none of the prior art pads effectively eliminate the aforementioned problems and furthermore, can exacerbate problems associated with slag emulsification, flow stagnation regions, thermal inhomogeneity, short-circuit flow, liquid residence distribution, and in particular, initial splash when the ladle is first opened.